This invention relates to a catalytic technique for upgrading a light aliphatic feedstream to heavier hydrocarbons. In particular, it provides a continuous process for oligomerizing a light olefinic gas feedstock, containing ethene, propene or other lower alkenes, to produce C.sub.4 + hydrocarbons, such as olefinic liquid fuels, isobutane, aromatics and other useful products. Ethene (ethylene, C.sub.2 H.sub.4) -containing gases, such as petroleum cracking offgas, are particularly useful feedstocks herein. A novel catalyst regeneration technique is used to dramatically reduce catalyst makeup rate.
Developments in zeolite catalysis and hydrocarbon conversion processes have created interest in utilizing olefinic feedstocks for producing C.sub.5 + gasoline, diesel fuel, etc. In addition to basic chemical reactions promoted by medium-pore zeolite catalysts, a number of discoveries have contributed to the development of new industrial processes. These are safe, environmentally acceptable processes for utilizing feedstocks that contain lower olefins, especially C.sub.2 -C.sub.4 alkenes. Conversion of C.sub.2 -C.sub.4 alkenes and alkanes to produce aromatics-rich liquid hydrocarbon products were found by Cattanach (U.S. Pat. No. 3,760,024) and Yan et al. (U.S. Pat. No. 3,845,150) to be effective processes using ZSM-5 zeolite catalysts. In U.S. Pat. Nos. 3,960,978 and 4,021,502, Plank, Rosinski and Givens disclose conversion of C.sub.2 -C.sub.5 olefins, alone or in admixture with paraffinic components, into higher hydrocarbons over crystalline zeolites having controlled acidity. Garwood et al. have also contributed to the understanding of catalytic olefin upgrading techniques and improved processes as in U.S. Pat. Nos. 4,150,062; 4,211,640 and 4,227,992. The above-identified disclosures are incorporated herein by reference.
Conversion of lower olefins, especially propene and butenes, over HZSM-5 is effective at moderately elevated temperatures and pressures. The conversion products are sought as liquid fuels, especially the C.sub.5 + aliphatic and aromatic hydrocarbons. Product distribution for liquid hydrocarbons can be varied by controlling process conditions, such as temperature, pressure and space velocity. Gasoline (C.sub.5 -C.sub.10) is readily formed at elevated temperatrue (e.g. up to about 400.degree. C.) and moderate pressure from ambient to about 5500 kPa, preferably about 250 to 2900 kPa. Olefinic gasoline can be produced in good yield and may be recovered as a product or fed to a low severity, high pressure reactor system for further conversion to heavier distillate-range products. Distillate mode operation can be employed to maximize production of C.sub.10 + aliphatics by reacting the lower and intermediate olefins at high pressure and moderate temperature. Operating details for typical "MOGD" oligomerization units are disclosed in U.S. Pat. Nos. 4,456,779; 4,497,968 (Owen et al.) and 4,433,185 (Tabak), incorporated herein by refernce. At moderate temperature and relatively high pressure, the conversion conditions favor distillate-range product having a normal boiling point of at least 165.degree. C. (330.degree. F.). Lower olefinic feedstocks containing C.sub.2 -C.sub.6 alkenes may be converted selectively; however, the low severity distillate mode conditions do not convert a major fraction of ethene. While propene, butene-1, and others may be converted to the extent of 50% to 95% in the lower severity moderate temperature distillate mode, only about 10% to 30% of the ethene component will be converted using HZSM-5 or similar acid zeolites. Many feedstocks of commercial interest, such as FCC offgas, dehydrogenation products, ethane cracking by-product, etc., contain both ethene and hydrogen along with H.sub.2 S and light aliphatics. Ethene can also be converted at moderate temperature with a bifunctional nickel catalyst.
U.S. Pat. No. 4,746,762 to Avidan et al., incorpoated by reference as if set forth at length herein, teaches a process for upgrading an ethene-rich olefinic light gas to a liquid hydrocarbon rich in olefinic gasoline, isobutane and aromatics by catalytic conversion in a turbulent fluidized bed of solid acid zeolite catalyst under high serevity reaction conditions in a single pass or with recycle of gas product. The process is particularly useful for upgrading FCC light gas, which usually contains significant amounts of ethene, propene, C.sub.2 -C.sub.4 paraffins and hydrogen produced in cracking heavy petroleum oils or the like. During such an upgrading process, the catalyst accumulates coke and is progressively deactivated. The deactivated catalyst is then regenerated by burning off the coke as the catalyst is fluidized in a stream of oxygen-containing inert gas at elevated temperatures typically ranging from 480.degree. C. to 705.degree. C.
Water is evolved from the combustion of coke during regeneration creating steam in the regenerator vessle. The medium-pore zeolite catalysts permanently deactivate upon contact with water at elevated temperatures at a rate believed to be proportional to an integral function of temperature and partial pressure of water. This phenomenon is commonly referred to as "steaming deactivation". Surprisingly, it has been found that coked catalyst from an olefin upgrading process such as described above can be regenerated at temperatures lower than previously believed possible. Further, such low temperature regeneration dramatically reduces steaming deactivation and extends the useful like of the catalyst. Fresh make-up catalyst represents a substantial portion of the total operating costs for a catalytic aromatization process. Consequently, longer catalyst life yields a significant savings in operating costs.